A couple of sig tries

First off try smaller sizes, most sigs are 350 by 150.
Second make your render smaller. Your render's are huge and pretty choppy, make them smaller by pressing CTRL+T and hold shift so it doesn't look weird afterwards.

First sig- Doesnt really blend and no text.
Second sig- Too smudgy and cant read text.

I went as easy as i could but otherwise good for your first tries. =]

 

I use Photoshop CS2, to lazy to update. Gimp has mostly the same controls though.

 

I haven't used GIMP for years now so I'm going to have to try to relate.

The first one: Like Kyle said, you may want to try a smaller size. Many use 350 x 150 as a starting point. I like to use 400 x 150. I would try to keep the width between 350 and 450 and the height should be between 100 and 150.

Your render on the first sig is really choppy. I would recommend finding renders on professional sites. GameRenders is a pretty good site for different renders. The render placement is okay. I would move it a little more towards the center but not on the center. For the type of sig you were going for, you should have a little bit of space between the left side of the sig and the render. You do, however, want to keep the space on the right side of the render greater than the left.

You chose fractals for your background. I don't particularly like fractals because they're very random. It's very hard to create a flow which is something you'll want to work on as you get better at making sigs.

The second one: This one is actually a lot better. Size is still an issue here but you actually created a flow. It's also good that you're realizing that not all sigs need a render. Some of the best sigs are done purely abstract. I like what you were doing with the text on this one, too.

I would recommend going to sites like DeviantArt and browsing through different GIMP tutorials and maybe look into downloading some brush packs for GIMP.

Good luck to you. PM me if you have any questions.

Edit: I just noticed that there's a sticky for Universal Resources that has just about everything you need under the Design category.